feat(vad): bundle Silero VAD ONNX model (2.3MB) in asr_agent/

Default path now resolves to silero_vad.onnx alongside worker.py,
falling back to VAD_MODEL_PATH env var.  Previously required manual
mount from /data/silero_vad/.

asr_agent/conversation_worker.py

@@ -196,7 +196,8 @@ class _TTSSegmenter:
 
 
 # ── VAD ──
-VAD_MODEL_PATH = os.environ.get("VAD_MODEL_PATH", "/data/silero_vad/silero_vad.onnx")
+VAD_MODEL_PATH = os.environ.get("VAD_MODEL_PATH") or os.path.join(
+    os.path.dirname(os.path.abspath(__file__)), "silero_vad.onnx")
 VAD_THRESHOLD_HIGH = float(os.environ.get("VAD_THRESHOLD_HIGH", "0.5"))
 VAD_THRESHOLD_LOW  = float(os.environ.get("VAD_THRESHOLD_LOW", "0.2"))
 VAD_WINDOW_SIZE    = int(os.environ.get("VAD_WINDOW_SIZE", "5"))